Prospects & Overviews Review essays Phosphatidylinositol 4,5-bisphosphate: Targeted production and signaling Yue Suny, Narendra Thapay, Andrew C. Hedmany and Richard A. Andersonà Phosphatidylinositol 4,5-bisphosphate (PI4,5P2) is a key Introduction lipid signaling molecule that regulates a vast array of bio- Phosphatidylinositol 4,5-bisphosphate (PI4,5P ) occupies a logical activities. PI4,5P can act directly as a messenger 2 2 central position in phosphoinositide signaling, as it can be or can be utilized as a precursor to generate other mes- used as a substrate to produce other second messengers, or sengers: inositol trisphosphate, diacylglycerol, or phos- can directly regulate a wide range of cellular functions (Fig. 1). phatidylinositol 3,4,5-trisphosphate. PI4,5P2 interacts with Nearly 60 years ago, Lowell and Mabel Hokin discovered the hundreds of different effector proteins. The enormous phosphatidylinositol (PI) cycle [1], and soon after a series of discoveries revealed that PI could be sequentially phosphory- diversity of PI4,5P2 effector proteins and the spatio- lated on its myo-inositol ring to generate PI4,5P2 [2–4]. At that temporal control of PI4,5P2 generation allow PI4,5P2 sig- time, PI4,5P2 was thought to only serve as an intermediate in naling to control a broad spectrum of cellular functions. the PI cycle [5, 6]. Early studies identified a role for PI4,5P2 as a PI4,5P2 is synthesized by phosphatidylinositol phosphate precursor of other signaling molecules, such as inositol tris- kinases (PIPKs). The array of PIPKs in cells enables their phosphate (IP3) and diacylglycerol (DAG) [6–8]. In the 1980s, targeting to specific subcellular compartments through the role of PI4,5P2 as a precursor was further expanded via the discovery that PI4,5P could be used as substrate for interactions with targeting factors that are often PI4,5P 2 2 PI3K to produce phosphatidylinositol 3,4,5-trisphosphate effectors. These interactions are a mechanism to define (PI3,4,5P3) [9]. spatial and temporal PI4,5P2 synthesis and the specificity In the mid 1980s, direct roles for PI4,5P2 signaling were of PI4,5P2 signaling. In turn, the regulation of PI4,5P2 defined. Anderson and Marchesi discovered that PI4,5P2 effectors at specific cellular compartments has implica- regulated the association of the cytoskeletal protein, band 4.1, with the membrane protein glycophorin, demonstrating tions for understanding how PI4,5P controls cellular 2 a role for PI4,5P in regulating the interaction of cytoskeletal processes and its role in diseases. 2 proteins with the plasma membrane [10]. Lassing and Lindberg discovered that PI4,5P2 directly interacts with, Keywords: and inhibits, the actin-modifying proteins profilin and gel- .lipid messenger; phosphatidylinositol 4,5-bisphosphate; solin, to promote actin assembly [11, 12]. These discoveries phosphatidylinositol phosphate kinase; PI4,5P effector 2 elevated PI4,5P2 from simply being a precursor of messen- gers, to a lipid messenger in its own right (Fig. 1), and initiated an explosion of research on PI4,5P2-binding proteins/effectors. Currently, hundreds of PI4,5P -binding DOI 10.1002/bies.201200171 2 proteins/effectors have been identified and this number is increasing. PI4,5P2 effectors are distributed to diverse sub- University of Wisconsin-Madison School of Medicine and Public Health, cellular compartments and mediate distinct biological Madison, WI, USA activities, including cell adhesion, cytoskeletal dynamics yThese authors contributed equally to this work. [13], cell polarity [14–16], secretion [17, 18], ion channel regulation [19], vesicular trafficking [20], nuclear signaling, *Corresponding author: and gene expression [21, 22] (Fig. 1). Richard A. Anderson E-mail: [email protected] There is now known to be a large and diverse array of PI4,5P2 effectors, many of which are found in the same mem- Abbreviations: brane compartment, e.g. the plasma membrane. This presents DAG, diacylglycerol; IP3, inositol trisphosphate; PI3,4,5P3, phosphatidylinositol a dilemma, the cellular or membrane contents of most second 3,4,5-trisphosphate; PI4,5P2, phosphatidylinositol 4,5-bisphosphate; PIPK, phosphatidylinositol phosphate kinase; PLC, phospholipase C. messengers dramatically fluctuate upon agonist stimulation Bioessays 35: 513–522,ß 2013 WILEY Periodicals, Inc. www.bioessays-journal.com 513 Y. Sun et al. Prospects & Overviews .... Figure 1. Overview of PI4,5P2 functions in the cytosol and plasma membrane. PI4,5P2 is a polyphosphoinositide that is phosphorylated on the 4th and 5th hydroxyl group on the myo-ino- sitol ring. Its greatest concentration is seen on the plasma membrane but it is also found on most cellular membrane compartments and in the nucleus. PI4,5P2 is utilized by PI3K or PLC to generate second messengers: PI3,4,5P3, DAG, and IP3. The generation of PI4,5P2 in a spatio-temporal manner is the basis for PI4,5P2 regulation of diverse cellular events, including endocytosis, exocytosis, vesicle trafficking, and cell migration. Review essays [6]. However, the cellular or membrane content of PI4,5P2 is areas, provides a synopsis of PI4,5P2 functions and how its relatively constant [23–25] and undergoes only modest signaling is modulated. changes upon stimulation [26, 27]. This raises the question: how does PI4,5P2 specifically modulate cellular events? Over the last few decades, the discovery and study of the phosphatidylinositol phosphate kinases (PIPKs), the PIPK isoforms synthesize PI4,5P2 at enzymes that produce PI4,5P2, revealed a mechanism for distinct cellular locations the targeted production of PI4,5P2. Distinct isoforms of PIPKs are targeted through association with unique interact- The PIPKI family is highly diverse. For example, there are at ing proteins to specific subcellular locations, and this con- least six PIPKIg splice variants expressed in humans, labeled trols the local production of PI4,5P2 [24, 27] (Fig. 2). Thus, PIPKIgi1 to PIPKIgi6 [36, 37] using the HUGO (Human Genome PIPKsalongwithenzymesthatconsumePI4,5P2,suchas Organization) nomenclature [36]. PIPKI isoforms all contain a phospholipase C (PLC), PI3K, and PI4,5P2 5-phosphatases, highly homologous kinase core domain with invariant cata- 2þ are critical for the spatial and temporal regulation of PI4,5P2 lytic residues that bind ATP or GTP and Mg , and residues levels [6, 13, 24, 28, 29]. that recognize the specific lipid substrate [27, 38, 39]. There are Six genes encode the PIPKs that generate PI4,5P2. These three domains in the PIPKI isoforms that are sequence diver- are the PIP kinases types I and II (PIPKI and PIPKII, respect- gent: the N-terminal, kinase insert and the C-terminal domains ively). PIPKI and PIPKII synthesize the majority of PI4,5P2 [24]. These regions of sequence divergence define cellular in the cell [27]. PIPKI preferentially phosphorylates the targeting and functional interactions [24]. The PIPKIg splice 5-hydroxyl position on the myo-inositol ring of PI4P to variants emphasize this concept, as these isoforms have generate PI4,5P2, while PIPKII uses PI5P as substrate and unique C-terminal sequence extensions from the PIPKIgi1 iso- phosphorylates the 4-hydroxyl position to produce PI4,5P2 form [36]. The diversity of these sequences confers the ability [27]. Both of the PIPK subfamilies have three isoforms a, b, to interact with different targeting proteins that are often and g, which are functionally diversified into splice variants PI4,5P2 effectors. These enzymes are then targeted to specific [24]. The PIPKs often directly interact with PI4,5P2 effector cellular compartments where they generate PI4,5P2 that proteins and this links PI4,5P2 synthesis to specific cellular specifically modulates effectors required for specific biological functions. In this way, targeted PI4,5P2 signaling modulates functions (Fig. 2). neuronal synaptic vesicle trafficking, epithelial morphogen- Consistently, each splice variant of PIPKIg localizes esis, cell migration, phagocytosis, nuclear events, and gene to select cellular locations. PIPKIgi2 can be targeted to expression [14, 16, 21, 30–32]. focal adhesions by an interaction with talin [40, 41]. This review focuses on the emerging roles of PI4,5P2 in Conversely, PIPKIgi4 localizes to nuclear speckles and the cytosol. PI4,5P2 function in nuclear events has recently PIPKIgi5 is found at endosome compartments [36]. For been reviewed [33, 34]. PI4,5P2 signaling or PI4,5P2-derived historical reasons, the nomenclature for PIPKIa and metabolites have been implicated in human disorders, includ- PIPKIb intheliteratureiscomplicatedbythefactthat ing mental retardation, bipolar disorder, schizophrenia, PIPKIa in mouse corresponds to PIPKIb in human and Alzheimer’s disease, diabetes, cancer, and ciliopathies [35]. vice versa. To avoid confusion, we only use the human Understanding the mechanistic roles of PI4,5P2 signaling in nomenclature when describing PIPKI isoforms in this human diseases is critical. This review, focusing on emerging review. PIPKIa is found at membrane ruffles [27] and nuclear 514 Bioessays 35: 513–522,ß 2013 WILEY Periodicals, Inc. ....Prospects & Overviews Y. Sun et al. Review essays Figure 2. The localized production of PI4,5P2 modulates specific Additionally, PI4,5P2-specific antibodies have been shown to PI4,5P2 effectors. Extracellular signals and regulators regulate the localize to the plasma membrane as well as to intracellular interaction of PIPKs with targeting factors that recruit PIPKs to compartments [45]. More recently, these antibodies were used specific subcellular